1. Technical Field
The present disclosure relates to a sound velocity correction device that corrects a sound velocity used in forming directivity of a sound in a direction toward a sound source from a microphone array device.
2. Description of the Related Art
In a monitoring system installed at a predetermined position (for example, a ceiling) in a factory, a store (for example, a retail store or a bank), or a public place (for example, a library), a wide viewing angle of video data (including a still image and a movie, the same applies hereinafter) in a predetermined range of a monitoring target is achieved by connecting a plurality of camera devices (for example, pan tilt camera devices or omnidirectional camera devices) to each other through a network.
Since the amount of information obtained is inevitably limited in monitoring only for a video, there is a strong demand for a monitoring system that obtains sound data in a direction in which a camera device captures a specific subject by disposing not only the camera device but also a microphone array device.
Hitherto, in a beam forming process (directivity forming process) for forming directivity in a specific direction with respect to sound data related to sounds collected by a microphone array device including a plurality of microphone elements, a sound velocity used to calculate a delay time necessary for the directivity forming process has been treated as a fixed value. For this reason, when a sound velocity changes due to, for example, changes in air temperature, the accuracy of directivity formed in the directivity forming process deteriorates.
Here, as the related art in which an adjustment value for setting a time difference between sound waves reaching microphones from a sound source to be zero is calculated using a temperature value of a propagation path of the sound waves from the sound source, for example, an image display device disclosed in Japanese Patent Unexamined Publication No. 2013-90289 is known.
The image display device disclosed in Japanese Patent Unexamined Publication No. 2013-90289 includes a microphone group, a sound source position calculation means, a test sound wave generation means, and an adjustment value calculation means. The microphone group includes a plurality of microphones for detecting sound waves emitted from a sound source. The sound source position calculation means calculates the position of a sound source on the basis of a time difference between sound waves reaching each microphone group. The test sound wave generation means are disposed at a position separated from the microphones at equal distances and emit white noise including sound waves having a plurality of different frequencies as sound waves. The adjustment value calculation means calculates a time difference between sound waves having a plurality of different frequencies reaching a microphone group for each frequency on the basis of the position of the test sound wave generation means which is calculated by the sound source position calculation means, and collectively calculates adjustment values for the respective frequencies for making phases of signals of sound waves detected by the microphone group conform to each other so as to set the time difference to be zero. Meanwhile, in Japanese Patent Unexamined Publication No. 2013-90289, an ambient temperature value used to correct propagation velocities of sound waves emitted from a sound source is a value which is input from a keyboard by a user.